PROJECT SUMMARY I propose to determine how loss-of-function of the TANK-binding kinase 1 (TBK1) kinase causes age- dependent degeneration in frontotemporal dementia (FTD), the second most frequent dementia after Alzheimer's disease. Heterozygous mutations were identified in the TBK1 gene as causative of familial and sporadic FTD and amyotrophic lateral sclerosis (ALS). Disease is likely to arise from haploinsufficiency as the majority of familial TBK1 mutations produce premature stop codons leading to non-sense mediated mRNA degradation. TBK1 is an IKK family kinase previously identified for a central role in controlling type- I interferon production in response to viral infection. It does this by phosphorylating interferon regulation factor 3 (IRF3), a transcription factor. TBK1 has also been implicated in regulating autophagy by its phosphorylation of the autophagy receptors optineurin (OPTN) and P62/SQSTM1, mutations in which cause FTD/ALS. Loss-of- function (LOF) variants in TBK1 causing FTD/ALS produce cytoplasmic TDP-43 aggregation, a hallmark of FTD instances without Tau or FUS pathology and in >95% of ALS. Patients that bear both a TBK1 mutation and another dominant disease-causing variant, such as GGGGCC repeat expansion of C9orf72 and point mutation in FUS, develop premature onset of ALS/FTD at ages of 40-50, suggesting that reduction in TBK1 activity may modulate aging of the central nervous system of synergize with other disease-causing mutation. In this proposal, I plan to conduct two independent but closely-related research aims to identify how TBK1 contributes to maintenance of a normal nervous system and how reduction in it contributes to age-dependent FTD and/or ALS. To identify mechanism(s) underlying toxicity in FTD and ALS mediated by reduced TBK1 activity, I will 1) determine whether TBK1 haploinsufficiency causes cognitive and/or motor deficits in aged mice; 2) determine whether reduction or deletion of TBK1 broadly from neurons across the brain causes cognitive or motor deficits; 3) determine whether deletion of TBK1 from motor neurons causes motor neuron deficits; 4) determine whether reduction in TBK1 kinase activity is a pathological feature of FTD (and ALS) in TDP-43, FUS, and C9orf72 mouse models; 5) determine whether toxicity from reduced TBK1 activity is non- cell autonomous using co-cultures of wild type, Tbk1+/-, or Tbk1-/- cortical neurons, astrocytes, and/or microglia. To identify the role of TBK1 in TDP-43 aggregation, I will 1) determine whether TBK1 kinase activity is activated by TDP-43 cytosolic aggregates 2) determine whether reduction in TBK1 or its activity affects clearance of TDP-43 aggregates, using an inducible TDP-43 aggregation system I have established in a neuronal cell line (SHSY5Y); 3) determine whether Tbk1 haploinsufficiency exacerbates deficits of TDP-43 transgenic mice.